Fruits, vegetables and other perishable items, including meat, poultry and fish, fresh or frozen, are commonly transported in the cargo box of a truck or trailer, or in an intermodal container. Accordingly, it is customarily to provide a transport refrigeration system in operative association with the cargo box for cooling the atmosphere within the cargo box. The transport refrigeration system includes a refrigerant vapor compression system, also referred to as a transport refrigeration unit, and an on-board power unit. The refrigerant vapor compression system typically includes a compressor, a condenser, an expansion device and an evaporator serially connected by refrigerant lines in a closed refrigerant circuit in accord with known refrigerant vapor compression cycles. The power unit includes an engine, typically diesel powered.
In many truck/trailer transport refrigeration systems, the compressor of the transport refrigeration unit is driven by the engine shaft either through a belt drive or by mechanical shaft-to-shaft link. More recently, all electric transport refrigeration systems have been developed for truck/trailer applications wherein the engine drives an on-board generator for generating sufficient electrical power to drive an electric motor operatively associated with the compressor of the transport refrigeration unit. For example, U.S. Pat. No. 6,223,546, assigned to Carrier Corporation, the same assignee to which this application is subject to assignment, the entire disclosure of which is incorporated herein by reference in its entirety, discloses an electrically powered transport refrigeration unit powered by an engine driven synchronous generator capable of producing sufficient power to operate the compressor drive motor and at least one fan motor. With respect to intermodal containers, clip-on power units, commonly referred to as generator sets or gensets, are available for mounting to the intermodal container, typically when the container is being transported by road or rail, to provide electrical power for operating the compressor drive motor of the transport refrigeration unit associated with the container. The genset includes a diesel engine and a generator driven by the diesel engine.
During transport of such perishable items the temperature within the cargo box of the truck, trailer or container must be maintained within strict temperature limits associated with the particular items being transported, regardless of potentially severe operating conditions imposed by the local environment in which the system is operating. For example when the transport refrigeration system is operated at high ambient temperatures and/or high altitude operation, the power demanded by the refrigeration unit at high cooling capacity demand may exceed the limited shaft power available from the engine, raising the potential for an engine stall or engine overload. In the event of an engine stall or engine overload, the loss of power from the generator will result in an undesired shutdown of the refrigeration unit.
In conventional transport refrigeration systems, the control system is open loop in that the system controller is unaware of the actual operating engine load. Rather, the transport refrigeration system controller uses algorithms that include safety margins to limit the engine shaft power demand in an attempt to prevent overload of the engine. However, at times, such as under aggravated service conditions and during transient operations, a lost in refrigeration unit performance and engine stalls or overload can still occur. A need exists for controlling the operation of the refrigeration unit in response to actual engine operating conditions so as to avoid engine stall or engine overload.